LeetCode Binary Watch

原题链接在这里：https://leetcode.com/problems/binary-watch/

题目：

A binary watch has 4 LEDs on the top which represent the hours (0-11), and the 6 LEDs on the bottom represent the minutes (0-59).

Each LED represents a zero or one, with the least significant bit on the right.

For example, the above binary watch reads "3:25".

Given a non-negative integer n which represents the number of LEDs that are currently on, return all possible times the watch could represent.

Example:

Input: n = 1
Return: ["1:00", "2:00", "4:00", "8:00", "0:01", "0:02", "0:04", "0:08", "0:16", "0:32"]

Note:

• The order of output does not matter.
• The hour must not contain a leading zero, for example "01:00" is not valid, it should be "1:00".
• The minute must be consist of two digits and may contain a leading zero, for example "10:2" is not valid, it should be "10:02".

题解：

上面4个表示小时，下面6个表示分钟，把小时向左移动6位 | 分钟就是二进制表示。二进制表示中 1 bit的个数等于num时就把这个组合加到结果中。

可以用Number of 1 Bits 或者Integer.bitCount()求出1 bit的个数.

Time Complexity: O(1), 12 * 60 * O(1). Space: O(1).

AC Java:

public class Solution {
    public List<String> readBinaryWatch(int num) {
        List<String> res = new ArrayList<String>();
        for(int h = 0; h<12; h++){
            for(int m = 0; m<60; m++){
                if(Integer.bitCount(h<<6 | m) == num){
                    res.add(String.format("%d:%02d", h, m));
                }
            }
        }
        return res;
    }
}